Process for electrowinning zinc from sulfide concentrates

ABSTRACT

This disclosure is directed to leaching zinc sulfide concentrates to facilitate the production of zinc and sulphur. solution. To provide for increased recovery the copper sulfate leach The zinc sulfide is leached with a copper sulfate solution, preferably under oxidizing conditions, to produce a copper sulfide and a zinc sulfate solution. The elemental sulphur can be obtained from the copper sulfide by a subsequent oxidizing leach and the zinc metal can be obtained by electrolyzing the zinc sulfate leach may be accomplished in a staging operation to establish an excess of zinc sulfide concentrate in the initial leach step.

United States Patent Renken et a1.

[54] PROCESS FOR ELECTROWINNING ZINC FROM SULFIDE CONCENTRATES [72]Inventors: Howard C. Renken; Theodoor W. Zegers, both of Columbus, Ohio[73] Assignee: Texas Gulf Sulphur Company, New York,

[22] Filed: May 19, 1969 [21] AppLNo; 825,578

[52] U.S.Cl ..204/114,23/114 [51] C22d 1/22, COlb 17/06 [58] FieldofSearch ..204/119, 114, 108;75/117, 75/120; 23/125, 135, 114

[5 6] References Cited UNITED STATES PATENTS 3,095,363 6/1963 Ruckwardtet a1 ..204/119 1,937,634 12/1933 Christensen ....204/119 1,890,93412/1932 Carson ..204/114 15] 3,655,538 [451 Apr. 11, 1972 1,553,4149/1925 Van Arsdale ..204/108 FOREIGN PATENTS OR APPLICATIONS 107,9827/1943 Sweden ..204/1 14 Primary Examiner-John I-I. Mack AssistantExaminer-R. L. Andrews An0rneyKenyon & Kenyon Reilly Carr & Chapin [5 7]ABSTRACT This disclosure is directed to leaching zinc sulfideconcentrates to facilitate the production of zinc and sulphur. solution.To provide for increased recovery the copper sulfate leach 8 Claims, 5Drawing Figures PATENTEDAFR 'l 1 I972 Beam/auras &

1 N VEN 'TORS BY m Zc-aaes 1 'PROCESSFORELECTROWINNINGZINCFROM SULFIDECONCENTRATES BACKGROUND acid.

3. Dissolution of the zinc oxide in dilute sulphuric acid to form -zincsulfate.

l. Purification ofthe zinc sulfate solution. 5. Electrolysisof the zincsulfate solution to produce zinc metal withth'e regeneration ofsulphuric acid.

Modificationsof this process have been sought toprovide elementalsulphur while maintaining a high yield of zinc metal.

Manytheoretical chemical equations can be drawn to illustrate methods bywhich zinc metal and elemental. sulphur couldbe extracted from zincsulfide. Foremost among these are the usesof sulphuric acid, underoxidizing and non-oxidizing conditions, to leachzincsulfide'concentrates. However these proposals-have not proven to beentirely satisfactory,

-due'either to low leaching rates, the requirements of complex processequipment or unattractive cost factors.

It is therefore an object of this invention to provide an improvedmethod of leaching zinc sulfide ores to recover values therefrom. p

lt is another object of the present invention to provide an improvedmethod for the recovery of elemental sulphur and -'zinc metal-from zincsulfide concentrates,

modification of the electrolyticzincprocess.

Qther objects andadvantages of the invention are set forthinconju'nction with the following detailed description.

THE INVENTION This invention is directed to the use of copper sulfate asa leaching agent for zinc sulfide concentrates, preferably underoxidizing conditions, to produce elemental sulphur and zinc metal.

The method of this invention includes first leaching the zincconcentrate under pressure by an exchange reaction with copper-sulfatesolution to give a zinc sulfate solution for electrolysis in accordancewith conventional methods and a copper-sulfide precipitate. Recovery ofelemental sulphur and rege'ne'ra'tionof the copper sulfate solution isaccomplished by a second pressure leach reaction of the copper sulfideprecipitate with spent electrolyte-under oxidizing conditions. In thisprocess, the copper is precipitated as cuproussulfide with theconcurrent production of sulfuric acid. To alleviate the-difficulties inpurification procedures caused by the sulfuricacid, the copper sulfateleach may altematively'be carried out-under oxidizing conditions.Furthermore, by staging the-operation to establish an excess of zincconcentrate in the initial copper sulfate leach, the amount-of acidproduction and soluble sulfate formation can'be minimized, withconsequent higher recovery. of elemental sulphur at'high zincextraction.

The invention is described in greater detail in conjunction withthefollowing figures and working examples in which:

FIG. -1 -is a process flow chart of the non-oxidizing coppersulfate-leach of zinc sulfide to produce-elemental sulphur andzincmetal.

FlG52 isa process flowchart of the oxidizing copper sulfate leach ofzinc sulfide to produce elemental sulphur and zinc metal.

FIG. 3 is a graph of the production of acid sulphur (based on totalsulphur) as a function of time in the process of FIG. 2.

F IG. 4 is a graph of the acidsulphur to zinc ratio as a functionoftimein the process of FIG. 2.

FIG. '5 is a graph of the calculated zinc extractions as a function oftime in the process of FIG. 2.

particularly by a In the process of FIG. 1 a zinc concentrate and coppersulfate solution are fed into a pressure leaching vessel 1. The zincconcentrate comprises predominantly zinc sulfide with minor amounts ofother naturally occurring constituents, such as iron sulfides. Thecopper sulfate solution is largely regenerated in thisproces's asreferred to in greater detail below.

The mole ratio of copper to zinc feed to vessel 1 is from about 1:1 to2:1, greater'amounts of copper may also be used. The preferred ratio ofcopper to zinc is about 1.5: l.

The temperature in the vessel is from about to 300 to 550F., preferably400 to 450F. The vessel itself is a corrosion resistant vessel, havingexternal temperature control means and an internal stirrer (neithershown). The reaction is carried out continuously with a normal residencetime of from about 1 to 7 hours.

The reaction product, which includes a copper sulfide and zinc sulfatesolution is withdrawn from the vessel 1 and passed through filtrationmeans 2. The filtration means is a conventional unit for the continuousseparation of a liquid-solid slurry. The impure zinc sulfate solution issent to a conventional electrolytic zinc process plant starting with thepurification procedure. The electrolytic zinc process is well known anddescribed in numerous texts, for example, Zinc The Science andTechnology of the Metal, Its Alloys and Compounds, C.

H. Mathewson, ed., pp. 174-225 (1959) and by Allen C.

Jephson et al. in Journal of Metals (18, 947-956 (Aug, 1966)). In unit 3the solution is purified, clarified, and electrolyzed to produce cathodezinc. The cathode zinc is smelted and cast into slabs which are storedas slab zinc in ware house 4 for shipment. Precipitates are keptseparated in storage means 5. The spent electrolyte or acid from theelectrolysis is sent to the pressure oxidation unit 6. Air, and theprecipitate from filtration means 2, which is largely a copper sulfide,are

concurrently fed to the pressure oxidation unit. This unit may besimilar in structure to vessel 1. The reaction products from the means 6are sulphur and a mixture of sulfates, predominantly copper sulfate withsome zinc sulfate formed from zinc sulfide. The sulfates are recycled tothe pressure leaching vessel l. The sulphur is purified and recovered ina conventional flotation means 7. 7

In this process copper sulfate solution wasfound to be an effectiveagent for leaching zinc sulfide concentrate. Experiments set forth belowestablished that a temperature of 400F gave satisfactory zinc leachingrates. At this temperature, zinc extractions of about percent after 4hours were obtained when using 1.5 moles of copper in the leach liquorfor every mole of zinc in the concentrate. At 450F, an extraction ofpercent was obtained under the same conditions. The presence of zincsulfate in the leach solution did not affect the reaction rate.

It was found that copper precipitates principally as cuprous sulfide (CuS), and not as cupric sulfide (CuS). Thermodynamic calculations alsopredict that cuprous sulfide is the stable form. The precipitation of CuS is accompanied by the formation of appreciable'amounts of sulfuricacid, which, however, is detrimental to maximum sulphur recovery. Thefollowing process minimizes this problem of acid formation.

FIG. 2 illustrates a preferred embodiment in which air or oxygen is fedto the pressure leach vessel 8 in addition to the zinc concentrate andcopper sulfate. The oxygen partial pressure may be from about 5 to psig,preferably from 10 to 50 psig. The reaction mixture is filtered in means9 and the solution therefrom is purified in means 10 with lime and zincdust in accordance with conventional procedures. The purificationprecipitates are stored in 11. The purified solution is electrolyzed inmeans 12, from which zinc metal is sent to storage 13. The spentelectrolyte, air, or oxygen, and the solids from the filtration means 9,are reacted in vessel 14. The pressure oxidation in 14 yields theregenerated sulfate solutions which are recycled into the pressure leachvessel 8. The residue from the pressure oxidation includes sulphur whichis purified in flotation unit 15 and zinc sulfide which is separated inflotation unit 16. The zinc sulfide is returned to the pressure leachvessel 8.

The above processes are illustrated further by the following examples.

EXPERIMENTAL PROCEDURE NON-OXIDIZING CONDITIONS The conditions underwhich copper sulfate solutions can be used effectively for the leachingof zinc sulfide concentrate are shown in the following experiments.

These experiments were conducted in a Pyrex vessel contained within aS-gallon autoclave and equipped with electrical resistance heating and acooling coil for automatic temperature control. A glass or tantalumstirrer was used, the latter being more suitable for higher pulpdensities.

The zinc concentrate used in the experiments was analyzed as follows:

Cu 0.73% Fe 8.2% S 32.1%

The copper in the concentrate was present as chalcopyrite (CuFeS Themajor part of the iron was found in a solid solution with zinc sulfide.

The main variables in these experiments were temperature, copper contentof leach solutions, and the presence of zinc in the leach liquor.

EXAMPLES 1-6 The effect of temperature on the reaction of zinc sulfidewith copper sulfate is shown in Examples l-6. In all runs, 50 g of zincconcentrate was leached with 500 ml aqueous copper sulfate solution ofindicated strength at various temperatures. The results of theseexperiments are listed in Table I.

n Zine contents of solutions were not determined because almost nocopper had precipitated W The results indicate that a minimumtemperature of 300F is required and that 400F is preferable to achieveacceptable zinc extractions in a reasonable time. The upper temperaturelimits are in part dictated by the equipment and economies. A suitabletemperature range is from about 300F to about 550F.

The results indicate that more copper is consumed than necessary for thestoichiometric formation of CuS. For the formation of Cu S, the molarratio of Cu precipitated to Zn dissolved is 1.6. The iron incorporatedin the ZnS lattice will react similar to zinc. Some of the iron in theconcentrate is combined with copper in chalcopyrite, CuFeS This mineralcent of the concentrate, is then incorporated in the sphalerite lattice.For every mole of zinc we therefore have 7.56/54.3 65.4/55.85 0.163moles of iron, reacting similarly. Every mole of zinc requires then1.163 X l.6=l.86l moles of 5 copper. Thecopper precipitate of the leachconducted at 400F (Table I, No. 6) should therefore not contain CuS. Thesame conclusion was arrived at from thermodynamic considerations.Examination of polished sections of residues from other leaches carriedout at 400F confirmed the absence of CuS.

EXAMPLES 7-15 In a series of leaches at 400F, the effects of time andinitial copper to zinc ratio are shown. Again 50 g of zinc concentratewas leached with 500 ml copper sulfate solution in most experiments;exceptions are clearly identified in Table II which indicates theresults.

TABLE II M01 ratio Mol ratio of of Cu to Zn dis- Cu preeip- Cu precipi-Time, Zn in solved, itated, tated to Zn In charge percent percentdissolved l 1. 63. 2 05. 5 1. 84 2 1. 51 73. 1 88. 8 I. 85 3 1. 51 77. 506.1 1. 86 4 1. 51 70. 3 90. 2 1. 87 4 1. 25 77. 8 100. 0 1. 4 1.38 75.6100.0 1.83 4 2.0 81.7 76.0 1.87 4 1.25 71.5 100.0 1.74 4 2.0 82.376.0 1. S5

' 5.3 grams H2504 added. 155 grams Zn-eoneentrate used.

Generally copper to zinc ratios of about 1.85 were obtained, indicatingprecipitation of Cu S. Exceptions are No. 11 and No. 14. In theseexamples the solution assays were confirmed by zinc assays of theresidues. In both cases copper was completely precipitated. It ispossible that under these conditions small quantities of CuS have beenformed.

Comparison of Nos. 13 and 15 shows no effect of increased pulp density.This was to be expected because the concentration of copper ions perunit surface area was not changed in these experiments by increasing thepulp density.

EXAMPLES 16-19 Actual process liquors contain zinc sulfate carried overfrom spent electrolyte through the pressure oxidation (see FIG. 1). Someexperiments were conducted which illustrated that this initial zinccontent of the leach solution does not have any effect on theextraction. In addition, a pulp density was chosen to produce pregnantsolutions suitable for normal zinc electrolysis practice. Previousexamples had illustrated that a change in pulp density does not affectthe rate of extraction.

In the earlier experiments at 400F, a molar ratio of copper in solutionto zinc in concentrate of 1.51 was used most frequently. For reason ofcomparison, the same ratio was used in these experiments, while zincsulfate was added to the leach solutions.

The data pertaining to these experiments are listed in Table 60 III.

These experiments show that the presence of zinc in the TABLE III Run N01G 17 1g 19 Solid e11arge 10 5;. Zn cone 100 5;. Zn eonc 100 g. Zn coneResidue from No. 18. Leaeli solution 4Synthetic 4Synthetic Pregnantliquor b from No 17 Synthetic. Time, hr 1 Temperature, 400... 450.

Zn extraction, per 79.4 85.

Fe extraction, percentot analyzed Cu in pregnant liquor, g.p.l 14.8 7.87Nil. 6.68. Molar ratio Cu precipitated to Zn dissolved 1.75 1.73 1.261.82.

a 500 m1. liquor containing 163 g.p.l. Cu and 57 g.p.l. Zn. b 407 ml.liquor containing 7.87 g.p.l. Cu and 149.0 g.p.l. Zn.

does not react with copper sulfate, and polished sections ofresidue donot show a sign of attack. Since the concentrate leach solution has nonoticeable effect on the extraction. An extraction of 79.4 percent wasobtained in experiment No. 16

contains 0.73 percent Cu, the amount of iron combined with it whichcompares with 79.3 percent for run No. 10 in Table 11,

in CuFeS, is 0.64 percent. The balance of the iron, 7.56 perwhere nozinc was present in the initial leach solution.

.cess of copper in theleach solution is desirable. In experiments No.17, No. 18 andNo. 19, it was shown that excess 1copper could berecovered in aleach in two stages. The excess copper in leach solutionNo. 17 was fully utilized in a l-hour leach withfresh zinc concentrate.As pointed out below, CuS

'is likely to be formed under these conditions. This is conifirmed bythe copper to zinc ratio obtained in experiment No.

18. If then all the copper from the pregnant solution of run No. 17 isprecipitated as CuS, an equal amount of copper will be required in runNo. 19 to convert CuS, without dissolving any .zinc. This accounts forthe higher copper to zinc ratio obtained inrun No. 19.

In the use of copper sulfate solutions for the leaching of zinc sulfideconcentrate, the followingreaction may occur to produce cupric sulfide.

Alternatively the precipitation of cuprous sulfide can take placeaccording to:

Measurements of the pH of leach liquors showed values generally below0.7 and pH values of 0.2 were common. This indicated that the reaction(2) takes place to a considerable extent. Under these conditions thefollowing reaction becomes important.

80,, H H50 3. Calculations show that at pH l, HSO, dominates stronglyover SO.

' The second reaction then should actually be written:

8Cu SZnS 4H O 3S0, 4Cu S 5Zn 4HS0 4H". 2'. The relative importance ofreactions (1) and (2) is shown by the following reaction:

8CuS 3Zn 4H O 350, 4Cu S 4HSO 4H 3ZnS 4. AF F -1 3,090 cal/mole formula,and

(HSO4-) (H+) 5 ZnH), one 1o These values show that at pH below about0.5, CuS is stable relative to Cu S provided ZnS is present. During thecopper sulfate leach, Cu S will first be precipitated on the ZnSparticles. By the time the acidity of the solution is such that CuScould be formed, all ZnS particles will be covered with Cu S. This.inhibits the formation of CuS according to equation 1 Therefore, whenthe leaching operation is carried out in two stages, where in the secondstage fresh zinc concentrate is contacted with pregnant liquorcontaining excess copper sulfate, the CuS would be formed.

EXAMPLE 20 The cuprous sulfide formed in thepressure leach can beleached'l with. acid under oxygen pressure to yield elemental sulphur.In: Canadian Pat. No. 712,989 granted to Sherritt GordonaMinesLimited,andincorporated herein by reference, a'processis described basedonthe reaction:

Although. this equation adequately described the overall reaction,calculations of thethermodynamics of the system indicateithatCus has tobe formed as an intermediate product.

Inu-Example20 a 100 g sample of copper sulfide precipitate (residue:run. No. 16, Table III) was leached with simulatedspentaelectrolyteunder 250 psig.air'at 225F for 2 hours. A residue'of.74.1 g was obtained containing a traceof elemental sulphur. Av polishedsection'of the residue revealed that the ori'ginalxcuprous sulfide wasalmost completely converted to cupric sulfide. lnthis stage of theoxidation process no elemental sulphur can .befonned.

In a continuation of this experiment, 42.8 g of the firsttreated residuewas mixed with 52.2 g fresh chalcocite (Cu S) and leached under the sameconditions. A residue of 62.0 g was obtained with 4.83 percent elementalsulphur.

This experiment shows that the oxidation of cuprous sulfide takes placein two steps. In the first step, cuprous sulfide decomposes into cupricsulfide-and copper ions. The formation of elemental sulphur from cupricsulfide takes place only after virtually complete conversion of Cu S.

As noted in Canadian Pat. No. 712,989 about 1 mole of sulphuric acid maybe used for each mole of copper. The oxygen pressure is from about 5 toabout l00'psig, and the temperature from about 175F to 225F. The Cu S isselectively oxidized at a rapid rate until about 95 percent of thecopper is extracted and the sulphur bound to it is converted toelemental sulphur. Present results indicate that about percent of thesulphur from decomposed cupric sulfide reports as elemental sulphur.

The copper sulfate formed in this leaching step is recycled to reactwith further zinc sulfide. The zinc sulfate which is formed is similarlyrecycled and as noted above has a negligible effect in the first leachstep in 1, from which it passes to the electrolysis unit.

The results obtained in Examples l-l9 by leaching zinc concentrate withcopper sulfate solutions were generally much better than the resultsobtained by leaching with sulfuric acid. Extractions of about 80 percentwere obtained after 4 hours at 400F with a copper sulfate deficiency. Aslight excess of copper resulted in about 82 percent extraction. Anextraction of 85 percent was achieved in a 4-hour leach at 450F in spiteof the copper deficiency. Thus extractions of about 80 percent arereadily obtained under proper conditions. Above this point, the rate ofextraction decreases because of the smaller surface area available forreaction.

Several alternatives are available to enhance the dissolution of zinc.One is to physically separate and regrind the coarse fraction prior toleaching. Alternatively, the unreacted zinc sulfide particles may beleft behind in the residue and recovered during subsequent treatment ofthe residue. In the copper sulfate regenerating step (pressureoxidation, FIG. 1), the copper sulfide layer will be removed from thezinc sulfide particles. By selectively removing the unreacted zincsulfide from the final residue by flotation, the zinc sulfide can berecycled, to result in high zinc recoveries. An additional advantage ofthis method is a beneficial effect on the sulphur recovery. The presenceof zinc sulfide in the leach residue would result in milder oxidizingconditions towards the end of the copper sulfate regenerating step. Thisinhibits the formation of sulfuric acid and this improves the yield ofelemental sulphur. Thus the presence of unleached zinc sulfide in theleach residue does not constitute a drawback for the overall process.This alternative is depicted in FIG. 2, but of course could also beemployed in the method of FIG. 1.

A disadvantageous factor in the process of FIG. 1 is inherent formationof sulfuric acid which was noted to occur during the precipitation ofcuprous sulfide. Because conventional procedures for purification of thepregnant solution prior to electrolysis require essentially neutralconditions, the use of a neutralizing agent might be required. Apractical solution is a change in the leaching procedure to ensureproduction of CuS rather than Cu S, as illustrated in FIG. 2. Theparameters for this process are illustrated in the following sequence ofexamples.

Zinc concentrate was leached with copper sulfate solutions attemperatures ranging from 300 to 450F. under oxygen pressures from 20 topsig. Copper precipitated initially as cuprous sulfide and leachsolutions contained considerable quantities of acid. Immediately afterprecipitation of cuprous sulfide, an oxidation reaction convertedcuprous sulfide to cupric sulfide under consumption of acid. This secondreaction was strongly promoted by high oxygen pressures while the firstreaction was promoted by high temperatures. Leach solutions obtained at400F. and 20 psig oxygen contained about 20 percent of the sulphur insolution as acid after 4 hours. Solutions from experiments at 300 F. and100 psig oxygen were essentially free of acid at any time.

With time cupric sulfide dissolved, especially under high oxygenpressures. Because of this reaction, from 33 to 65 percent of thesulphur in the zinc concentrate was solubilized. The results indicatethe advantages of a staging operation to combine high zinc extractionwith low sulphur dissolution and an essentially acid free solution. Thuswith .a low zinc extraction, obtained by using an excess of zincconcentrate, the resulting leach solution is well suited for solutionpurification prior to zinc electrolysis. Elemental sulphur is producedfrom cupric sulfide in the residue, iron is rejected and excess zincconcentrate is recycled. This process provides for higher zincrecoveries than possible in the conventional roasting-electrolysisprocess and is suitable for treating middlings and other copper-zincsulfide products.

EXPERIMENTAL PROCEDURE OXlDlZlNG PROCEDURES The zinc concentrate used inthe following examples was analyzed with the following results: Zn 51.2percent; Cu 0.96 percent; Fe 8.3 percent; and S 32.0 percent.

The experiments were performed in an Autoclave Engineers S-gallonautoclave that was modified by placing a tantalum reaction vessel withina steel sleeve holder. The space between the steel sleeve holder and theautoclave wall was filled with water. A cooling coil was immersed in thewater. This arrangement made it possible to cool the reaction mixturerapidly after termination of an experiment without exposing to coil tothe corrosive leach liquors. The autoclave was heated to the desiredreaction temperature by electric'resistance heating; constanttemperature was maintained by automatic control. During the experimentsthe slurry was vigorously agitated by a tantalum stirrer. Pressurizedair was introduced into the solution through stainless steel tubing withits opening approximately 2 inches above the stirrer blades. Pressurewas maintained at the desired level by a pressure regulator on the aircylinder. To avoid oxygen depletion during the reaction, gas wasconstantly removed from the autoclave at a rate of approximately 1.25liters per minute (atmospheric pressure and room temperature). In eachexperiment the solid charge consisted of 200 grams of zinc concentratewhich would require about liters of oxygen to react completely accordingto Reaction (1). Consequently sufficient oxygen for complete reactionwas provided every hour.

Slurry samples were taken at regular intervals. For this purpose a valvein the air supply line was closed momentarily and a second valve betweenthe first one and the autoclave was opened to the atmosphere. Thepressure in the autoclave forced a slurry sample up through the airinlet tube into a graduated glass cylinder. in most experiments twosamples of 100 ml volume each were removed during a run.

EXAMPLES 21-26 A series of six leaching experiments was performed inwhich the temperature was either 300F, 350F, or 400F, and the partialpressure of oxygen above the solution was 50 psig or 100 psig asindicated below. In all experiments, 200 grams of zinc concentrate wasleached with one liter of solution containing 95.5 grams copper ascupric sulfate. The amount of copper present in the leach solutions was105 percent of the amount required to dissolve all of the zinc and theiron associated with it in the zinc lattice as calculated from thereactron:

5ZnS 4Cu 5Zn 4CuS S0,," and the analogous reaction for dissolution ofiron.

The first experiment was terminated after 3 hours. The five otherexperiments each lasted 6 hours and slurries were sampled after 1 hourand after 3 hours.

Solution samples were analyzed for Zn, Cu, Fe, and S.

After the experiments were terminated, the slurries were filtered, theresidues washed and dried and the filtrates combined with washings madeup to 2 liters. Solutions and residues were both analyzed for Zn, Cu,Fe, and S. The results of these experiments are presented in Table IV.

TABLE IV 200 grams of concentrate, 1 liter solution, (1 hours except N0.1

Experiment No 21 22 23 2t 25 21;

Temperature, F 300 300 350 400 300 350 Oxygen partial pressure,

p.s. .g 100 100 100 100 50 50 Solution after 1 inn, g.p.i

Zn 18. 3 27. 1 39. 5 16. 2 27. (i 63. 7 41.7 14.0 57.5 35.7 2. 07 1. 743. 26 2. 24 2. 38 42. 8 40. 7 40. 3 39. 8 39. 7

40.7 35.0 41.4 39. 7 33. 8 29. 6 20. 1 27. 7 17. 5 17. 8 2. 96 2. 24 1.58 3. 15 3. 88 S 38. 4 32. 2 40. 9 32. 2 30. 8 Final residue weight, g156.0 98. 7 132. 7 109. 3 137. 8 150.0 Residue analysis, percent:

Zn 33. 8 17. 2 19.1 18. 8 15. 3 20.3 21.0 29. 0 36.0 36. 8 39. 3 37. 77. 69 11.0 9.29 12.6 7.52 7. 09 S 30.1 25.9 27.1 24.1 29.0 28.4 Zincextraction, percent. 49. 5 83.8 75. 1 76. 5 80. 3 71. 1

QLBased n B liters solution. N.S.=no sample.

Observation of solution samples and residue weights indicated that thebest elimination of copper had been achieved within the first 3 hours ofRun No. 24, at 400F, and that some benefit could be expected from loweroxygen pressures. Accordingly further experiments were conducted atrelatively high temperatures and low partial pressures of oxygen. Theleaching time was reduced to 4 hours and the solution was sampled after1 and after 2 hours. The results are discussed in further detail below.

EXAMPLES 27-30 Three experiments were performed at 400F and 20 psigoxygen, and one at 450F, and 20 psig oxygen. In all experiments 200grams of zinc concentrate was used. initial solution volume was oneliter in three experiments, and 400 ml in the other run.

Results of these experiments are shown in Table V. Zinc extractions forall experiments were based on the distribution of zinc between solutionsand final residue.

TABLE V 200 grams concentrate, 20 p.s.i.g. oxygen, 4 hours, 1 litersolution except as noted Experiment N o 27 28 29 30 Temperature, F 400450 400 400 First sample b solution, g.p.1.:

Zn 25.2 40.7 41.4 N.S.

" 400 ml. solution. After 1 hour for Runs 7 and alter hour for Run No..1. e After 2 hours for Run No. 7; after 1% hours for Run No. 9. Basedon distribution of zinc between solids and solutions. N.S.=no sample.

Most leach solutions contained acid, evidence of formation of cuprotmxulfide. This was also confirmed by sulphur deficicncicn in the residuesif all the copper was assumed to be present as cupric sulfide. Theamount of sulphur as acid inbeen formed by Reactions (2) and (7). Hence,the increase of sulphur in leach solutions was initially determined bythe stoichiometry of Reaction 2). Because iron reacted proportionally tozinc, the increase in sulphur under these conditions creased wiiiiincreasing p l r and ranged from about 3 was directly proportional tothe amount of zinc dissolved. In percent at 300 F to about 20 percent ofthe total amount of ll experiments h amount f copper in solution hadsulphur m so utlon at 4 F- decreased sharply by the time the firstsample was taken. Thus most Predominant i'eaciioiis f i Piace dumigReaction (9) did not occur to any measurable extent in the ii s i gcggga tigg sgi g under Conditions p y in first hour. On this basis minimumvalues for the solution i volumes at the time of first sampling werecalculated. SZnS 18 $3236 2 2:; f g 'zg 'g Similarly maximum values werecalculated assuming Reaction 5ZnS z+ ICu 20 q SZX 1' 46:8 S Z- (9) didproceed at a constant rate throughout the entire run. 2 4 The zincextraction curves are shown in FIG. 5. It appeared I m F3 a g i A S t Rthat Reaction (9) could indeed be neglected for the first hour a l s i ee c e en y a Ogou o of each experiment. Approximate zinc extractions atsubt sampling times, were calculated by assuming Reac- SFeS 8cu++ 4H,o5Fe+ 401 s 1+ are 50,-. f

tion (9) to proceed at a constant rate. 10. Part of the dissolved ironsubsequently precipitated according It has Shown that by choice ofleaching condl to one or more of the followin reactions tlons, cupricsulfide can be obtained as the condensed reac- 4Fe++ O 10" 4Fe(O'H) 1+8H+ H tion product without fon'nation of appreciable quantities of4Fe+Aq +022+ 45042: 2H2O 4"Fe(OI nso4 l acid. To achieve adequate zincextractions however it was the reaction time which caused redis- 4Fe A?411+ 0 660 2Fe (SO 2n,o. ,3. necesary 9 In View of the Small quantitiesinvolved any acid formation solution of cupric sulfide. ThlS isequivalent to loss of sulphur due to precipitation of iron would benegligible. The presence from the a i it a gt iz resissoiigion of acidin the leach solutions is thus attributed to a cupric i e on Y Occur? aer a Coiisi era 6 P predominance of the acid forming Reaction (2) overthe acid Zinc had b'eeii dissoivedy Providing sufficient Zinc consumingReaction 7 fide area, 1.e., by using sufficlent excess ZlllCconcentrate, the

The amount of sulphur found in final leach solutions in- 3O i q i y 0fredissoiuiioii of cupric Sulfide would be dicated that Reaction (9),dissolution of cupric sulfide, took mlmmlled- The molar w o n to CuShould be greater than place. If Reaction (2), and (7), only take place,only 16 to 20 iii and p y from 1251 to i a P to percent of the sulphurin zinc concentrate should have been Alternatively when lower Yams of Zn9 Cl! are Used, as dissolved during leaching. However, 33 to 65 percentwas acfrom 1:1 to 1:2, the reaction should be carried to about to tuallydissolved. 35 80 percent conversion, preferably from to percent con-Table VI shows the dissolution of sulphur from zinc concenversion, basedupon the extraction of Zn from the ZnS. The trate for the variousexperiments. Also shown are the percentpregnant solution of such aleaching operation is essentially ages of sulphur converted to acid. N Ai free of copper and acid, and suitable for solution purification TABLEIv Actual S Zinc Theoretical B dissolution, S conversion Oxygen cxtrac-S dissolution, percent of S to acid, percent pressure, tion, percent ofS in in conof total S in p.s.i. percent concentrate centrate solution100 83. a 19. a 60.9 2. 6 100 75. 1 17. a 43.8 9. 2 100 76. 6 17.6 62. 61a. 1 50 80.3 18.5 40.4 3.6 60 71. 1 16. 4 35. a s. s 20 72. 4 16. 6 33.0 20. 2 20 86. 6 19. 9 64. 6 16. 7 20 71.6 16. 5 as. 1 19. u 20 77. 917. 9 47. 5 6. 5

' Assuming no redissolution of 0118.

in onclitersolutiou.

It can be seen that up to 400F, dissolution of sulphur was largelycontrolled by oxygen pressure. Best sulphur retentions were obtained at400F and 20 psig oxygen pressure. The experiments at higher oxygenpressures indicate a minimum sulphur extraction at 350F. Thus sulphurextraction would have been lower at 350F. and 20 psig than at 400F and20 psig oxygen. Zinc extraction, however, was relatively low in bothexperiments at 350F, and quite large quantities of acid were formed. InFIG. 3, acid formation is shown as a function of time for allexperiments where samples were taken. In all cases the amount of acidformed was at a maximum at a time prior to termination of theexperiment. This indicates that Cu S was the primary copper precipitateand that CuS was formed by subsequent oxidation. FIG. 4 shows the ratioof acid sulphur versus zinc as a function of time. The generalsimilarity of the corresponding curves in FIG. 3 and 4, indicates thatthe decline in acid content of the solutions was caused by the acidconsuming Reaction (7) and not by an increase in total sulphur contentaccording to Reaction (9). The data presented in Table VI show, however,that Reaction (9) did occur. Reaction (9) can only take place after CuShas prior to zinc electrolysis. The leach residue, containing cupricsulfide, oxidized iron and excess concentrate can be leached with returnacid under conditions conducive to the formation of elemental sulphur.The resulting solution is suitable for leaching additional zincconcentrate. The solids can be separated by physical methods e.g.,flotation, into three fractions; elemental sulphur, zinc concentrate,and iron reject. Zinc concentrate is of course returned to the coppersulfate leach. This shows the importance of excess of zinc concentratein the initial stage to avoid dissolution of cupric sulfide. The use ofexcess concentrate in the first stage of the process is thereforeconsidered a preferred method of operation.

This process, outlined in FIG. 2, provides higher zinc extractions thanpossible with the conventional roasting-electrolysis process, becausethe roasting step with its inherent formation of insoluble zinc ferritesis eliminated. The process can also be integrated with a conventionalinstallation for treatment of middling fractions from the flotationcircuit. Another application is the treatment of bulk flotationconcentrates or treatment of concentrates from ores that cannot beeffectively separated by selective flotation.

This invention has been described in terms of specific embodiments setforth in detail. Alternative embodiments will be apparent to thoseskilled in the art in view of this disclosure, and accordingly suchmodifications are to be contemplated within the spirit of the inventionas disclosed and claimed herein.

We claim:

1. The process which comprises mixing zinc sulfide ore and a coppersulfate solution at an elevated temperature and pressure to form acopper sulfide and a zinc sulfate solution,

separating said copper sulfide from said zinc sulfate solution,

purifying said zinc sulfate solution, electrolyzing said zinc sulfatesolution to form metallic zinc and sulfuric acid, mixing said coppersulfide with oxygen and said sulfuric acid at elevated temperatures andpressures to form copper sulfate solution and sulphur, separating saidcopper sulfate solution therefrom and mixing therewith fresh zincsulfide ore to repeat said process.

2. The process of claim 1 wherein the mole ratio of zinc to copper isfrom 2:1 to 1:2.

3. The process of claim 1 wherein the mole ratio of zinc to copper is1:1 to 1:2 and the conversion of zinc sulfide to zinc sulfate is from 40to 80 percent.

4. The process which comprises mixing zinc sulfide ore and coppersulfate solution and an oxygen containing gas in a first pressureleaching vessel, discharging the reaction mixture to a filtering means,withdrawing the pregnant solution from the filtering means and treatingsaid pregnant solution in a purification and electrolysis means toproduce zinc metal and a spent sulfuric acid electrolyte, charging saidsulfuric acid, air and the solids from said filtering means to a secondpressure leaching vessel, regenerating copper sulfate solution in saidsecond leaching vessel, recycling said copper sulfate solution to saidfirst pressure leaching vessel, discharging the residue from said secondleaching vessel to a sulphur flotation means, withdrawing sulphur fromsaid flotation means, withdrawing unreacted zinc sulfide from saidflotation means and recycling said zinc sulfide to said first pressureleaching vessel.

5. The process of claim 1 in which the amount of oxygen in said firstleaching vessel is about 20 to 100 psig.

6. The process of claim 1 wherein the ratio of zinc sulfide to coppersulfate in said first pressure leaching vessel is in excess of the molarquantities needed for reaction.

7. The process of claim 1 wherein the ratio of Zn to Cu in said firstpressure leaching vessel is 1:1 to 1:2 and the conversion of zincsulfide to zinc sulfate therein is from 40 to percent.

8. The process of claim 7 wherein the charge to said second leachingvessel is unreacted zinc sulfide, a copper sulfide, and about 1 mole ofsulfuric acid per mole of copper.

2 g UNITED STATES PAT ENT omen CERTIFIC ATE 0F CORRECTION Patent No. vDated April #1972 I Inv n C's) Howard C. Renken and Theodoor 'W.Ze'gers- It is certified that error appears in the above-identifiespatent and that said Letters Patent are hereby corrected as shown below:

' In the Abstract, line 2, delete "solution."

In the Abstract, line 3, delete the entire line 3 V In the Abstrzatct,line 9, after "sulfate" insert solution. To providefor increasedrecovery the copper suifate-e- Col. 6, lihe.50,' change "this" ,to thusCol. 7, line 33, after "ing" change "to" to the Col. 9",""iW8fi Tnsert2+7j Col. 9, line 22, change "new" to we Col. I 9, line 23; change "l+Fe*TA?" to 4Fe Signed and sealed this 25th day of July 19-72.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOI'TSCHALK Attesting Officer 1Commissioner of Patents

2. The process of claim 1 wherein the mole ratio of zinc to copper is from 2:1 to 1:2.
 3. The process of claim 1 wherein the mole ratio of zinc to copper is 1:1 to 1:2 and the conversion of zinc sulfide to zinc sulfate is from 40 to 80 percent.
 4. The process which comprises mixing zinc sulfide ore and copper sulfate solution and an oxygen containing gas in a first pressure leaching vessel, discharging the reaction mixture to a filtering means, withdrawing the pregnant solution from the filtering means and treating said pregnant solution in a purification and electrolysis means to produce zinc metal and a spent sulfuric acid electrolyte, charging said sulfuric acid, air and the solids from said filtering means to a second pressure leaching vessel, regenerating copper sulfate solution in said second leaching vessel, recycling said copper sulfate solution to said first pressure leaching vessel, discharging the residue from said second leaching vessel to a sulphur flotation means, withdrawing sulphur from said flotation means, withdrawing unreacted zinc sulfide from said flotation means and recycling said zinc sulfide to said first pressure leaching vessel.
 5. The process of claim 1 in which the amount of oxygen in said first leaching vessel is about 20 to 100 psig.
 6. The process of claim 1 wherein the ratio of zinc sulfide to copper sulfate in said first pressure leaching vessel is in excess of the molar quantities needed for reaction.
 7. The process of claim 1 wherein the ratio of Zn to Cu iN said first pressure leaching vessel is 1:1 to 1:2 and the conversion of zinc sulfide to zinc sulfate therein is from 40 to 80 percent.
 8. The process of claim 7 wherein the charge to said second leaching vessel is unreacted zinc sulfide, a copper sulfide, and about 1 mole of sulfuric acid per mole of copper. 